Content-based education programmes, in which a second/foreign language (L2) is used as the medium of instruction when teaching non-language content subjects, aim at both content and L2 learning. With such dual goal in mind, and with the rapid expansion of the programmes to contexts where students might have only basic L2 proficiency, there have been consistent calls for reconsidering the roles of first language (L1) in the teaching and learning process. The functions of L1 in content-based classrooms have been well documented, but it is necessary to have a more systematic approach to planning and using L1. This paper seeks to address this gap by applying the notions of 'curriculum genres' and 'task structure' when analysing patterns of teachers' use of L1 and L2 in a collection of content subject lessons observed in English-medium secondary schools in Hong Kong. With illustrative episodes presented in this paper, we would argue that 'curriculum genres' and 'task structure' can serve as useful tools for both researchers and teachers to analyse and critically reflect on patterns of pedagogic practices and language use in content-based classrooms. These will have significant implications for future research on using L1 and illuminate effective pedagogy in content-based education.
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